What is lap dip?

Simply:

A lab dip is a swatch of fabric test dyed to hit a colour standard.

To know more:

Several lab dips may be submitted for feedback until the standard is achieved and the lab dip is approved. Lab dips are reviewed in a "light box". A light box is a machine where the interior is painted a neutral colorless grey (as to not skew the color of the dip) and has several options of light under which you can see your dip. Usually a company will select one light source (for example "daylight") as it's standard so both factory and home office see the lab dips under the same source. The person responsible for reviewing lab dips will then give comments such as "too dark, too red, make lighter, more green" until the standard is achieved. There is also now a machine used to read lab dips called a "spectrometer" which will give the same feedback. As not everyone sees color as well as the next (and you can be tested to see how well you see color) a machine can take out human error.